/Engine/Source/Runtime/Landscape/Public/LandscapeSubsystem.h
Can be called at runtime : (optionally) flushes grass on all landscape components and updates them
void RegenerateGrass ( bool bInFlushGrass, bool bInForceSync, [TOptional](API\Runtime\Core\IO\TOptional)< [TArrayView](API\Runtime\Core\Containers\TArrayView)< FVector >> InOptionalCameraLocations )